The present invention relates to detector log video amplifiers and more particularly to detector log video amplifiers having improved recovery times.
As is known, a detector log video amplifier (DLVA) produces a video frequency output signal having an amplitude logarithmically proportional to the power level of a radio frequency (RF) signal applied to an input port thereof. An extended range DLVA responds to RF signals over a large power range, such as 60 dB (for example, between -40 dBm and +20 dBm). Conventional extended range DLVA's comprise a pair of RF detectors coupled in parallel to the input port of the DLVA and having differing operating RF power ranges. For example, the first detector may respond to RF power levels between -40 dBm and -20 dBm while the second detector responds to power levels between -20 dBm and +20 dBm. The pair of detectors produce video-frequency signals in response to the RF signals applied thereto. The video signals typically are amplified by a series of linear amplifiers, coupled through a set of logarithmic amplifiers, and combined in a summing amplifier to produce a video output signal having an amplitude proportional to the logarithm of the RF input signal power.
Typically, the RF signal is applied as a series of pulses to the input port of the DLVA. The recovery time of the DLVA is defined as the time interval between the falling edge of an initial video voltage output pulse produced in response to an initial RF input pulse and the rising edge of the next succeeding video voltage output pulse (produced in response to the next succeeding RF input pulse) with such next succeeding video pulse voltage being within a specified range (typically, .+-.0.5 dB) of its nominal voltage when the initial video pulse is not present. Recovery time increases with increasing DLVA RF input signal power and pulse width (up to about 10 .mu.sec). Thus, in conventional DLVAs a directional coupler (e.g., a 20 dB coupler) is provided having an input coupled to the DLVA input port. The coupled path of the directional coupler is fed to the second RF detector (i.e., the detector responding to RF power levels between -20 dBm and +20 dBm at the DLVA input port), while the low-loss (i.e., throughput) path of the directional coupler is applied through an RF limiter to the input of the first RF detector (i.e., the detector responding to RF power levels between -40 dBm and -20 dBm at the DLVA input port). Thus, RF input pulses at the high end of the specified range (e.g., -40 dBm to +20 dBm) never exceed 0 dBm at the input of the second detector. The RF limiter responds to the power level of the DLVA RF input signal fed thereto via the directional coupler and typically begins attenuating such signal when the power level of such signal exceeds approximately +4 dBm, and limits the power level of the RF signal applied to the first RF detector as the power of the DLVA RF input signal further exceeds +4 dBm, thereby coupling an RF signal having a power level between +7 dBm and +12 dBm to the first RF detector when the RF signal at the DLVA input port has a +20 dBm power level.
While such a DLVA has performed satisfactorily and produced acceptable recovery times (for example, 10 .mu.sec for +20 dBm, 1 .mu.sec input pulses) in some applications, in other applications it is required to provide reduced recovery times from those achievable with such a DLVA. For example, to provide a recovery time of 1 microsecond (.mu.sec) or less, power levels at each RF detector input cannot exceed 0 dBm for longer than 1 .mu.sec. At such power level and duration, the RF detector and succeeding linear amplifiers are incapable of dissipating the excessive power applied thereto in less than 1 .mu.sec after the initial RF pulse is removed, and a successive RF input pulse cannot be accurately processed by the DLVA until such power has been dissipated. As discussed, in the DLVA described above, the power applied to the first detector is as much as +7 dBm to +12 dBm at an RF input power of +20 dBm--far exceeding the 0 dBm threshold and thus increasing recovery time above 1 .mu.sec.
One DLVA providing improved recovery times utilizes frequency compensation of the linear amplifiers to alter the frequency response of such amplifiers. With such arrangement, circuit components (i.e., the value of resistors, capacitors, etc.) are selected to reduce the DLVA recovery time (such as about 3 .mu.sec for 1 .mu.sec, +20 dBm input pulses) but such improved recovery time still exceeds that required in some applications (such as less than 1 .mu.sec).
In another DLVA, an RF preamplifier is disposed at the input of the RF limiter, and an attenuator is inserted between the output of the RF limiter and the input of the first RF detector. The gain of the preamplifier and the loss of the attenuator are selected to be equal. Such arrangement limits the power level coupled to the first detector to less than 0 dBm, thereby reducing recovery time below 1 .mu.sec. However, the preamplifier and attenuator reduce the "flatness" of the video output signal over the operating frequency range of the DLVA and also cause an increase in the current consumption of the DLVA.